A power source supplies a current at a voltage to a load for a period of time. Characteristics of the load typically define the kind of power source that is appropriate. Electronic circuits may demand a relatively small current for an extended period of time. A mechanical device may demand a short burst of relatively high current to generate a powerful motion. Some loads, like an electric vehicle may require smaller currents for motion over a flat surface and a larger current to move up an incline.
Electric powered vehicles may employ large lead acid batteries to provide energy for their traction systems and operating systems. A battery of this type typically delivers from 24 to 48 volts. A traction system may be powered to move an electric powered vehicle around the workplace under the control of an operator or a computer. Traction systems may draw large currents from the DC bus during acceleration or when moving up an incline, but normally demand lower currents for extended periods of time. Operating systems, such as a lift system, may consume a significant portion of the stored power during normal truck operation. When lifting heavy loads, the operating systems may demand large currents for short periods.
A conventional lift truck will typically operate from 5 to 6 hours on a fully charged battery. When the battery voltage drops below a certain level the truck is driven to a battery station where the depleted battery is removed and a fully charged replacement battery is installed. This operation typically requires from 20 to 30 minutes during which the truck and operator are nonproductive.
Efforts have been made to improve the vehicle designs, particularly in ways that will increase the productive period of the battery. For example, the battery may be recharged during truck operation by an alternator, generating charging currents with motions of the traction and lift systems. While this approach does recover some of the energy, lead acid batteries are inefficient energy recovery devices. A large portion of the regenerated energy is dissipated as heat and lost. Periods when large currents are drawn during truck operation significantly limit battery life.
As can be seen, there is a need for power sources capable of providing large currents in short bursts and lower currents over an extended period of time. A hybrid power source consisting of a high power source and a high energy source can result in a high energy and as well as high power device when the load duty cycle of each component power source is actively managed. For example: a fuel-cell, which is a high energy density device, may be hybridized with a supercapacitor, a high power device, to construct such a source. A supercapacitor or ultracapacitor is an electrochemical capacitor that has an unusually high energy density when compared to common capacitors.